1. Field of the Invention
The invention relates to aqueous polymer dispersions based on polyurethanes, a process for their preparation and their use.
2. Description of the Prior Art
Adhesives based on polyarethanes are predominantly solvent-containing adhesives which are applied to the two substrates to be bonded and are dried. By subsequently joining the two substrates under pressure at RT or after thermal activation, a bond build-up with a high initial strength is obtained immediately after the joining operation.
For ecological reasons there is an increasing demand for suitable aqueous adhesive dispersions which can be processed to corresponding aqueous adhesive formulations. Such systems have the disadvantage that after evaporation of the water the initial heat resistance immediately after the joining operation is significantly lower compared with solvent-containing adhesives, in spite of prior thermal activation of the dry adhesive film.
The use of silica products for various applications is known from the prior art. While solid SiO2 products are often employed as fillers or adsorbents for controlling rheological properties, the use as a binder of diverse inorganic materials, as a polishing agent for semiconductors or as a flocculation partner in colloidal chemical reactions dominates in the case of silica sols. For example, EP-A 0 332 928 discloses the use of polychloroprene latices in the presence of silica sols as an impregnating layer in the production of fireproofing elements. FR-A 2 341 537 and FR-A 2 210 699 describe pyrogenic silicas in combination with polychloroprene latices for the production of flame-resistant foam finishes or for bitumen improvement, and JP-A 06 256 738 describes them in combination with chloroprene/acrylic acid copolymers.
The present invention was based on the object of providing aqueous adhesive compositions which, after application to the substrates to be glued and joining, have a high initial heat resistance, especially after thermal activation.